Gardner File No. 23 -d-
Rangeland soil carbon and
nitrogen 'responses to grazing
J.T. Manley, G.E. Schuman, J.D. Reeder, and R.H. Hart
ABSTRACT: Land manag~rs. liv~stock producas, and th~ public ar~ conc~m~d about th~ ~ffict1
ofgrazing on soil quality and sustainability of rang~!4nd mourm. Pastum at th~ High P!4ins
Grasslands &uarch Station n~ar Chryenn~, ~oming. graud fOr th~ past 11 y~ars at a h~avy
stocking rat~ (67 st~~r-dayslha) untkr thn:~ manag~ment systems, wa~ compan:d to continuous
light grazing (22 ste~r-dayslha) and to liv~stock aclosum. Th~ h~avy stocking rate mulud in
slightly 1m than 50% utilization of th~ annualfOrag~produmi, a kv~1 r~commended by !4nd
manag~ment agenci~s. Prior to initiating this grazing r~uarch th~ rang~land had not b~en graud
fOr about 40 y~ars. Soil organic carbon and nitrogen r~spons~ wa~ roaluaud by colkcting soil
sampks to 91 em (36 in) depth. Soils had high~r amounts ofcarbon and nitrogen in th~ surfau
30 em (12 in) on th~ graud pastum compar~dto nativ~ rang~!4nd wha~ liv~stock w~n: aclud­
ui. Howron; soil carbon and nitrogen b~low 30 em was simi!4r among ali grazing tr~atments.
Carbon and nitrogen dynamics wa~ gr~aUJt in th~ JUrfac~ 30 em wha~ mon: than thr~~-fOurths
ofth~ plant root biomass aists. Grazing strategi~s and stocking rat~s impoudfOr th~ past 11 y~ars
on this mix~d grass prairi~ did not detrimentally affia soil organic carbon and nitrogen kv~ls.
Th~ data, in fact. sugg~st that responsible grazing enhanud th~ ovaall soil quality as asms~d by
there parameters.
esearch on grazing lands has historically
focused on the effects of v:uious man­
agement practices on forage production and
animal response; little attention has been
given to the impact of grazing on the nutri­
ent dynamics of soils. Recent interest in
"soil health" or "soil qualiry" has directed at­
R
u:nti0i"l rlJ
gi'~zing
impact.> \:;n sc-il parCiffie­
ters that make up soil health/qualiry. Few
studies have evaluated the effectS of grazing
on soil organic matter and irs relationship to
water and nutrient cycling, and related plant
producciviry. In general, the available data
do not indicate any single or consistent re­
sponse of soil organic carbon (C) and nitro­
gen (N) to grazing. Some research has re­
ported increases in both soil organic C and
N (Dormaar et aJ. 1990; Dormaar and
Willms 1990a; Ruess and McNaughton),
while other studies have found no response
in soil organic C and N to grazing (Kieft;
Mathews et al.; Milchunas and Laurenroth).
Smoliak et al. reported that increased graz­
ing pressure on mixed prairie resulted in de­
creases in needleandthread (Stipa comata)
and an increase in blue gram (Bouteloua gra­
cilis), which has a greater shallow root mass;
hence, soil organic carbon increased. Bauer
et al. found that grazing reduced soil organ­
ic C and increased soil organic N. These inj. T. Man/ry is a graduate research assistant. G.E,
Schuman and ].D. &eder an: soil scientistJ. and
R.ff Hart is a rangei4nd scientist. &zngeland Re­
sourCfi Raearch Unit. U.S. Department ofAgri­
culture-Agricultural Research Service. Cheyenne.
WY82009.
j. Soil and Waur Cons. 50 (3) 294-298
294
consistent findings demonstrate the com­
plex interaction between grazing and soil or­
ganic matter.
Many factors determine the response of
soil organic matter ro livestock grazing.
Floate summarized these factors as (a) the
initial status of vegetation and soil, (b) en­
"Iiro!!!!!enr::d f:!.c!o!~, especially !!!oistt'.re
and temperature, and (c) grazing history
(intensity, frequency, duration, and type
of animal). Soils with inherently low soil
organic C are more prone to change in re­
sponse to grazing (Dormaar et al. 1977)
than soils high in organic C. Precipitation
and temperature limit above-ground bio­
mass production and soil organic matter
(Parton et al.), and influence rates ofliner
decomposition and nutrient cycling
(Charley). Year-to-year fluctuations of en­
vironmental conditions may affect differ­
ences in biomass production as much as
grazing (Ashby et al.; Milchunas and Lau­
renroth).
Grazing intensity, frequency, and dura­
tion all affect the plant communiry and
the magnitude and direction of change in
soil organic C and N. Light grazing can
result in greater species diversity and pro­
duction compared to areas where livestock
grazing is excluded Oohnston). On the
other hand. high intensity and/or early
season grazing have been found to have a
greater negative impact on litter and live
plant biomass than light intensiry and/or
late season grazing of mixed prairie pas­
tures in Canada (Naeth et al.). Moderate
grazing of mixed grass prairie in North
Dakota resulted in greater litter decompo­
JOURNAL OF SOIL AND W,ITER CONSERVATION
sition and soil N mineralization man ei­
ther heavy or no grazing (Shariff et al.).
Most research on this subject has com­
pared soil C and N concentrations in me
surface soil of grazed pastures and non­
grazed exclosures. However. animal graz­
ing has been reported to increase soil bulk
density (Bauer et al.; Dormaar and
Willms 1990b; Pluhar et al.) and increases
in soil bulk density will greatly affect me
results if soil C and N concentrations, nor
toral mass, are reponed. greatly over-stat­
ing the response to grazing treatments
(Simpson et al.). Soil organic C and N
should therefore be reported based on
mass. However. not everyone agrees how
to best express and evaluate management
effects on C and N changes in soil. Skene
suggests that the use of soil bulk densiry
to normalize data may lead to misinter­
pretation. He and other researchers
presently assessing soil C and N dynamics
are suggesting mat soil volume be consid­
ered and that comparisons be made on
equal soil volumes even though soil depth
may vary because of bulk density. Addi­
tienally. since root biomass and distribu­
t?on affect soil organic matter content and
distribution, soil profiles should be sam­
pled sufficiently deep to ensure that a ma­
jority of the rooting depth is included in
the assessment.
The objective of our research was to
~11:m· ~~e effeCTS of!;everal erazine man­
agement strategies on soil or~ic -C and
N after 11 years on a semi-arid mixed
grass prairie. Grazing management strate­
gies evaluated included zero, light, and
heavy grazing intensities and season-long.
rotationally-deferred, and shon-duration
rotation grazing.
Study sites and methods
Study sites. The research was conduct­
ed at the High Plains Grasslands Research
Station near Cheyenne. Wyoming. The
native rangeland is a mixed-grass prairie,
with roIling topography and elevations
ranging from 1910 to 1950 m (6266 to
6397 ft). The climate is semi-arid with an
annual frost-free period of 127 days and
mean (1871-1986) annual precipitation
of 338 mm (13.3 in), of which 70% oc­
curs from April 1 mrough September 30.
Dominant soil series are Ascalon and Alt­
van sandy loams [mixed, mesic, Aridic
Argiustoll (Stevenson et al.)].
Dominant cool-season grasses are west­
ern wheatgrass (Pascopyrum smithi,) and
needleandthread and the dominant warm­
season grass is blue grama. A survey of the
plant community prior to initiating the
grazing study indicated the range sites of
the area were in good condition.
Reprinted from the Journal 01 Soil and Water ConsefVafion
May"",-,une 1995. Volume 50 Number 3
CopYright Q 1995 Soil and Water Conservation SOCI9'Y
Prior to establishment of the grazing
management phase of i:he research in
1982, the area had not been grazed by do­
mestic livestock for about 40 years. Graz­
ing strategies, grazing intensities, and pas­
ture design are shown in Figure 1. Soils
were sampled where grazing management
was continuous season-long at a light
stocking rate (22 steer-days/ha), and rora­
tionally deferred, short-duration rotation,
and continuous season-long, all at a heavy
stocking rate (67 steer-days/ha). The
heavy stocking rate resulted in slightly less
than 50% utilization of the annual pro­
duction. Further details of the grazing
strategies are given in Hart et al. Before
the grazing research started, permanent
50-m (160 ft) line transects were located
on near-level (0-6%) sites in each pasture
on the Ascalon soil series. The experimen­
tal design of the grazing study is a ran­
domized block design with two replicate
blocks (Hart et al.).
Soil sampling. In July 1993, soil sam­
ples were collected from the continuous
season-long, light stocking rate (CL); con­
tinuous season-long heavy stocking rate
(CH); shorr-duration rotation, heavy
stocking rate (SH); rotationally deferred,
heavy stocking rate (RH) pastures, and
the exclosure (EX). Soil sample sites were
established at 10m (32 ft) intervals along
the: perina";:": 50 in (160 ft) line trAr;·
sectS that had been established for evaluat­
ing plant community response to the
grazing treatments. Five cores were taken
on each transect to a depth of 30 cm (12
in) and separated into 0-3.8, 3.8-7.6, 7.6­
15, and 15-30 cm (0-1.5, 1.5-3, 3-6, and
6-12 in) increments. Surface plant litrer
was carefully removed from the area
where soil samples were taken. Cores were
taken with a hydraulic soil sampling ma­
chine with a soil tube diameter of 4.6 em
(2 in). Because of soil moisture limita­
tions, the deeper soil cores [30-91 em
(12-36 in)] were not taken until May
1994. Soil cores were taken to the 91 cm
depth, the surface 30 cm increments re­
moved, and samples collected for the 30­
46,46-61 and 61-91 cm (12-18, 18-24,
24-36 in) depths. The individual soil
sample increments were placed in sealed
plastic bags in coolers and transported to
a constant temperature room [5'C
(41'F)] until analysis. 'Upon return to the
laboratory the samples were broken up by
hand and plant crowns and visible roots
removed. Duplicate soil cores were col­
lected at the second and fourth sampling
locations on each of the transectS for de­
termining bulk density (Blake and
Hartge) (Table 1).
Sample analysis. The soil samples were
Table 1. 5011 bulk density of various grazing management strategies at varying soil
depths
Soil Depth
EX
CL
RH
SH
1.01'
1.36
1.39
1.39
1.39
1.39
1;14
1.36
1.26
1.26
1.26
1.26
1.17
1.33
1.38
1.38
1.38
1.38
1.11
1.27
1.42
1.43
1.43
1.43
cm
0-7.6
7.6-15
15-30
30-46
45-61
61-91
EX=exclosure, CL=continuous light, CH=continuous heavy, RH=rotationally deferred heavy.
SH=short-duration rotation heavy
'values are a mean of four replicate samples, two samples taken on each of two transects
.
PASTURE DIAGRAM
...".:..,-.
..- - _. .
.......
. ..
'.
,'s,. '.
,,-
Rep I
eM
I~~~~~~~~~~RM~.~.~.~.~==:.:--:=-=-~
----------­
t=
Rep 11
a.Smile
•
Stocking rates
Grazing strategies
L -Ught
M -Moderate
H =Heavy
C - Continuous
R = Rotationally
deferred
S .. Short Duration
rotation
EX Exclosure
Water source
=
Figure 1. Diagram of grazing systems and stocking rates study, showing replications,
grazing strategies, and pasture design
dried in forced air ovens at 60'C (I40'F)
to a constant weight. The dried samples
were ground to pass a 2-mm (0.08 in)
screen. Soil organic N was determined on
a subsample by the micro-Kjeldahl
method described by Schuman et al. The
digest was analyzed for ammonia by con­
tinuous flow colorimetric analysis (Tech­
nicon TRAACS 800). Soil organic C was
determined utilizing a modification of the
Walkley-Black procedure (Nelson and
Sommers). The modification was a con­
version from a colorimetric endpoint to a
millivolt endpoint (Raveh and Avnim­
elech). Soil organic C was determined on
a subsample of the soil ground to 0.50
mm (0.02 in).
Data analysis. Soil organic C and N
concentration, for each transect and by
depth increment, were convened [0 a
mass basis (kg/ha) using the mean bulk
density of the two soil cores obtained
from each line transect. The bulk density
data in Table 1 are presented for assessing
C and N changes on a concentration basis
because of the debate surrounding data
presentation on a mass basis. Analysis of
variance was used to evaluate treatmem
effects on soil organic C and N and least­
significant-difference (LSD) procedures
were used for mean separation (Steel and
Torrie). All statistical analyses were evalu­
ated at P ~ 0.10.
Results and discussion
Soil organic C and N in the surface 7.6
cm (3 in) of soil were significantly lower
in the exclosure compared to all of the
grazing management strategies (Figures 2
and 3). No differences existed between
any of the grazing strategies, except thar
in the 3.8-7.6 em (1.5-3 in) soil depth the
organic C was significantly lower under
the continuous-light grazing strategy than
"l,\ y. I UN E I <J q ~
295
25
DEX .CL .CH.RHC3SH
a ab
ab
-20
o
o
o
or-
x 15
ro
.c
0)
~
-10
()
o
(J)
5
o
Figure 2. Soil organic carbon as affected by various grazing strategies (exclosure, con­
tinuous-light, continuous-heavy, rotatlonally-deferred heavy, and short-eluratlon rotation
heavy grazing), 0-30 cm soli depth
Bars within a soil depth increment with the same letter are not significandy different. P S 0.10.
25
DEX .CL .CH.RHIiiillSH
a
I
x
co
:E
15
0)
~
Z
::E 10
tti
"0
Q;
~ 5
o
Figure 3. Soil organic nitrogen as affected by various grazing strategies (exclosure, con­
tinuous-light, contlnuous-heavy, rotationally-deferred heavy, and short-eluratlon rotation
heavy grazing), 0-30 cm soil depth
Bars within a soil depth increment with the same lettcr arc not significandy diffcrent. P S 0.10.
under any heavily grazed management,
bur was still significantly greater than in
the exclosure. In rhe 7.6-30 cm (3-12 in)
soil depth, differences in soil organic C
and N contents among grazing srraregies
were more varied, but C and N contents
296
in the exclosure remained lower than, or
at best comparable to, C and N under the
various grazing treatments. These data ex­
hibit rhe importance of the surface few
centimeters of soil as it relates to C and N
dynamics under a grassland. Seventy to
JOURNAL OF SOIL AND WATER CONSERVATION
90% of the root biomass is present in the
surface 30 cm (I2 in) of the soil in rhis
type of grassland system (Oormaar et al.
1984); therefore, nutrient availability in
this zone is very important. In rhe 30-91
em (12-36 in) depth, soil organic C and
N contents were not significandy differ­
ent among ungrazed and grazed treat­
ments (Figure 4). Soil organic C and N
concentrations were quite low and vari­
able at this depth in the soil. Only a small
percentage of the root system is present in
this soil depth increment; therefore, it is
nor surprising that grazing for 11 years
had no affect on the lower porrion of rhe
soil profile.
The higher levels of soil organic C and
N in the surface soil of the grazed range­
land may be due in part to the effects of
grazing on litter and standing dead com­
ponents of the above-ground biomass. Al­
though grass roots are the primary source
of organic matter in rangeland soils,
above-ground litter provides a secondary
source (Aandahl). The grazing treatments
essentially eliminated the standing dead
component and greatly reduced the sur­
f:tce litter component. Animal traffic may
be enhancing physical breakdown and soil
incorporation of this residual plant mater­
ial. In comparison, 70% of the above­
ground phyromass in the exclosures was
in the form of litter and standing dead
plant materiaL We ~stim:l[e rhat this large
component of recalciuant marerial immo­
bilized about 35 kg N/ha (31 Ib N/ac)
and 1550 kg Clha (I ,383 lb Clac) above­
ground in the exclosures'. Species compo­
sition shifts within a plant community
can also result in C changes as discussed
earlier (Smoliak et al.). However, our re­
search did not show an increase in blue
grama (Hart et al.) nor did it show a sig­
nificant below-ground biomass response
to grazing2.
In general, defoliation is thoughr ro
stimulate an increase in allocarion of C
and N ro regrowing leaves and a decrease
in allocation of C to roots (Oeding et al.),
resulting in reduced root biomass (Oor­
maar er al. 1990; Johnston) and reduced
root exudate contributions to soil organic
matter. Thus researchers have predicred
losses rather than gains, of soil organic C
and N with grazing (Holland et al.). How­
ever, multiple harvests (simulated grazing)
raken throughout the growing season have
been shown to result in greater plant pro­
duction than a single estimate obtained ar
peak standing crop (Murz and Orawe).
Dodd and Hopkins found thar clipping
'G.E. SchurruuJ. 1994. unpublished data.
'R.H. Hart. 1994. unpublished data.
6
.---------,-------------.80
O'EX . C L .CH.RHrnmJSH
05
o
-
-
o
60 oo
o
....
><4
-
-
ns
.c
)(
CV
~3
40 .s:::.
C,
-
o
z
~
:22
ns
20°
.tIJ
"C
Gi
~1
o
o
Nitrogen
Carbon
Figure 4. Soil organic nitrogen and carbon as affected by various grazing strategies (ex­
closure, contlnuous-llght, contlnuous-heavy, rotatlonally-deferred heavy, short-duratlon
rotation heavy grazing), 30-91 cm soli depth
No significant differences berwttn grazing m:aunenu wimin a par.une[ee weee obsecved. P ~ 0.10.
blue grama five times gave greater produc­
tion man clipping mree or four times, in­
dicating mat mis species would respond [Q
grazing defoliation and produce greater
aboveground biomass. Grazing also stimu­
lates (illering (Floate) and rhizome pro­
duction (Schuman et al. 1973). Thus, if
plant growth is stimulated by grazing,
then greater CO 2 sequestration would
occur. which could result in greater C allo­
cation to below-ground portions of the
system. Dyer and Bokhari (Dyer and
Bokhari) inferred that blue grama re­
sponded [Q defoliation by rapidly translo­
eating material to (he crowns and roots
and by increasing root respiration and/or
root exudation rates. Ruess and Mc­
Naughton (Ruess and McNaughton) sug­
gested that grazing accelerates the rate of
nutrient cycling by stimulating primary
production and net nutrient flux, mereby
increasing the percentage of the system's
nutrients that are available and which
cycle rapidly near me soil surface.
The resultS of this study indicate that
responsible grazing strategies implemented
11 years earlier did not detrimentally af­
fect soil organic C and N levels in me ac­
tive and important upper 30 cm (12 in) of
the soil profile under native mixed-grass
rangeland. In fact, the data indicate that
grazing enhanced me overall soil quality as
assessed by these parameters, and that
plant production should be sustainable.
REFERENCES CITED
Aandahl. A.R. 1981. Soils of [he Great Plains: Land
Use. Crops and Grasses. University of Nebraska
Press. uncoln. NE.
Ashby, M.M.. R.H. Hart, and J.R. Forwood. 1993.
Plant community and cattle tesponses to fifty
yc:ars of grazing on shongrass prairie. US DA­
Agricultural Research Service. Rangeland Re­
sources Research Unit. RRRU-1. Cheyenne.
WY.
Bauer, A.• C.V. Cole. and A.L. Black. 1987. Soil
propetty comparisons in virgin grasslands be­
rween grazed and nongrazed management sys­
tems. Soil Science Society of America Journal
51:176-182.
BI:t.ke, G.R.. and K.H. Hartge. 1986. Bulk density­
core method. In: A Klute (ed.) Methods of soil
analysis. Parr 1. 2nd ed.• Agronomy Monograph
No.9. American Sociery of Agronomy. Madi­
son. WI.
Charley. J.L. 1977. Mineral cycling in rangeland
ceosysterrn. In: R.E. Sosebee et al. (eds.) Range­
land Plant Physiology. Range Science Series
4:214-256.
Detling. J.K.• M.1. Dyer. and D.T. Winn. 1979.
Net phorosynthesis. root respiration and re­
growth of Bou[e1oua gracilis following simulated
grazing. Oecologia 41:127-134.
Dodd. J.D.• and H.H. Hopkins. 1958. Yield and
carbohydrare content of blue grama as affected
by clipping. Transactions of the Kansas Acade­
my ofScience 61 :280-287.
Dormaar, J.F.• A. Johnston. and S. Smoliak. 1984.
Seasonal changes in carbon content. and dehy­
drogenase. phosphatase. and urc:ase acrivities in
mixed prairie and fescue grassland Ah horizons.
Journal of Range Management 37:31-35.
Dormaar. J.F.• A. Johnsron. and S. Smoliak. 1977.
Seasonal variation in chemical characteriStics of
soil organic marter of grazed and ungrazed
mixed prairie and fescue grassland. Journal of
Range Management 30:195-198.
Dormaar. J.F.• S. Smoliak. and W.O. Willms. 1990.
Distribution of niuogen fractions in grazed and
ungrazed fescue grassland Ah horizons. Journal
of Range Management 43:6-9.
Dormaar. J.F.• and W.O. Willms. 1990a. Effect of
grazing and cultivation on some chemical prop­
erties of soils in the mixed prairie. Journal of
Range Management 43:456-460.
Dormaar. J.F.. and W.D. Willms. 1990b. Sustain­
able production from [he rough fescue prairie.
Journal of Soil and Water Conservation 45: 137­
140.
Dyer. M.1.. and U.G. Bokhari. 1976. Plant-animal
interactions: Studies of [he effects of grasshop.
per grazing on blue grama grass. Ecology
57:762-772.
Floare. M.J.S. 1981. EffectS of grazing by large hee·
bivores on nitrogen cycling in agricultural
ecosystems. In: F.E. Clark and T. Rosswall (cds.)
Terrestrial Nitrogen Cycles. Ecological Bulletin
33:585-601.
Han. R.H .. M.J. Samuel. P.S. Test, and M.A.
Smith. 1988. Catde, vegetation. and economic
responses to grazing systems and grazing pres­
sure. Journal of Range Management 41:282·
286.
Holland. EA. W.J. Panon. J.K. Deding. and D.L.
Coppock. 1992. Physiological responses of plan[
populations to herbivory and their consequences
for ecosystem nutrient flow. The American Nar­
uralist 140:685-706.
Johnston. A 1961. Comparison of lightly grazed
and ungrazed range in the fescue grassland of
southwestern Alberta. Canadian Journal of Planr
Science 41 :615-622.
Kieft. T.L 1994.. Grazing and plant.canopy effects
. on semiarid soil microbial biomass and respira·
tion. Bioi. Fen. Soils. 18:155-162.
Mathews. B.W.• LE. Sollenberger. V.D. N"m. and
c.R. Staples. 1994. Impact of grazing manage·
ment on soil nitrogen. phosphorus. potassium
and sulfur distribution. Journal of Environmen·
tal Qualiry 23:1006-1013.
Milchunas. D.G .• and W.K. Laurenroth. 1993.
Quantitative effectS of grazing on vegetation and
soils over a global range of environmentS. Eco·
logical Monographs 63: 327-366.
Mutt. J.L.. and D.L. Drawe. 1983. Clipping fre­
, 'quency and fertilization influence herbage yield
and crude protein content of 4 grasses in Soum
TCX2S. Journal of Range Management 36:582­
585.
Naeth. M.A.• A.W. Bailey, D.J. Plu[h. D.S.
Chanasyk, and R.T. Hardon. 1991. Grazing im·
pacts on litter and soil organic matter in mixed
prairie and fescue grassland ecosystems in Alber·
ra.Journal of RanRe Management 44.7-12.
Nelson. D.W.• and LE. Sommers. 19!1i. Tora! cac­
bon. organic carbon. and organic marrero In:
AL. Page. R.H. Miller. and D.R. ~eney (cds.)
Methods of soil analysis. parr 2. 2nd cd.. Agron.
Monograph No.9. American Sociery of Agrono­
my. Madison. WI.
Parron. W.J.• D.S. Schimel. C.V. Cole. and D.S.
Ojima. 1987. Analysis of facrors controlling soil
organic marter levels in Great Plains Grasslands.
Soil Science Sociery of America Journal
51: 1l73-1179.
Pluhar. J.J .• R.W. Knight. and R.K. Hei[schmidr.
1987. Infaltration rares and sediment production
as influenced by grazing systems in the T CX2S
rolling plains. Journal of Range Management
40:240-243.
Raveh. A. and Y. Avnimelech. 1972. Potentiometric
determination of soil organic marter. Soil Sci­
ence Sociery ofAmerica Proceedings 36:967.
Ruess. R.W .• and S.J. McNaughton. 1987. Grazing
and the dynamics of miuient and energy regular.
ed microbial processes in the Serenge[i grass­
lands. Oikos 49:101-110.
Schuman. G.E.. M.A. Sranley. and D. Knudsen.
1973. Auromated tora! niuogen analysis of soil
and planr samples. Soil Science Sociery of Amer­
ica Proceedings 37:480-481.
Schuman, G.E.• D.T. Booth, and J.W. Waggoner.
1990. Grazing reclaimed mined land seeded ro
native grasses in Wyoming. Journal of Soil and
Water Conservation 45:653-657.
Shariff. A.R.. M.E. Biondini. and C.E. Grygiel.
1994. Grazing intensiry effectS on litter decom­
position and soil nitrogen mineralization. JOU[­
nal Range Management 47:444-449.
Simpson. J.R.. S.M. Bromfield. and O.L. Jones.
1974. Effects of management on soil ferriliry
under pasture. 3. Changes in [ora! soil niuogen.
carbon. phosphorus. and exchangeable cations.
Australian Journal of Experimental Agriculture
MAY - J U N E I?? 5
297
and Animal Husbandry 14:487-494.
J.K.M. 1966. Errors in ..coetion studies of
soil organic mauer. Journal ?f the Australian Institute Agricultural Science 32:208-209.
Smoliak. S.• J.F. Dortn2af. and A. Johnston. 1972.
Long-term gtazing effects on Stipa-Boutdoua
ptairie soils. Journal of Range Management
25:246-250.
Steel. R.G.D .. and J.H. Torrie. 1980. Principles
and procedures of statistics; a biometrical approach. 2nd d .. MaGraw-Hill Book Co.. New
York. NY.
Stevenson. A.• R.E. Baumgartner. and G.E. Schuman. 1984. High Plains Grasslands Research
Station. Detailed Soil Survey. USDA-Agricultural Research Service. Cheyenne. WY and
USDA-Soil Conservation Service. Casper. WY;
and Wyoming Agricultural Experiment Station.
Laramie. 1-841103.62.
Technicon TRAACS 800. 1987. Crude protein in
feeds. Industrial Method No. 825-87r. Technicon Industrial Systems. Tarryrown. NY.
S~ne.
298
10 LJ RNA L 0 F SOl LAN 0
W A TE ReO N S E R V A TI 0 N